When you hear something that is so profound, brilliant, or novel, that it blows your mind.

Guy 1: "... and thats how the theory of evolution works."
Guy 2: "Holy shit man, I think I just had an intellectual orgasm"

Teacher: "So I was reading through these economics tests last night when I came across Sam's example of a vineyard as one of the best positive externalities. It was so brilliant I had an intellectual orgasm."